Shadow Catcher is Lighter than Rest of Scene
I came across this thread when searching for a solution to the same exact problem as the OP. I tried various fixes, included the "simplified" node set up mentioned above by cegaton. Nothing worked.

The node setup posted by bmanner is the same node setup I get when I use a shadow catcher plane in Blender 2.79b (Mac version). I'm a rank beginner with nodes, so reducing the three "inputs" down to the two shown in the fix was not working for me.

Through all the playing around, however, I did find a very simple fix. By checking the "clamp" checkbox in both of the "add" nodes, everything renders properly. The shadow catcher no longer brightens everything it touches. No other aspect of the scene seems to be affected in the render. No idea why or how this works, but it does. 

I realize this is an old thread. Since I haven't seen this solution anywhere else, I figured this would be a good place to mention it. The OP's screen shot is attached, with the boxes noted. I hope this helps someone.